1. Field of the Invention
The present invention relates generally to an apparatus for calibrating the displacement of reflective parts in a diffractive optical modulator, and, more particularly, to an apparatus for calibrating the displacement of reflective parts in a diffractive optical modulator, which is capable of measuring variation in the displacement of upper reflective parts using a portion of diffracted light having a plurality of diffraction orders and then compensating for the measured variation in the displacement of the upper reflective parts.
2. Description of the Related Art
Active research into various Flat Panel Displays (FPDs) has been conducted in order to develop next generation display devices. Of them, generalized FPDs include Liquid Crystal Displays (LCDs) using the electro-optic characteristics of liquid crystal, and Plasma Display Panels (PDPs) using gas discharge.
LCDs are disadvantageous in that the viewing angle thereof is narrow, the response speed thereof is slow, and the manufacturing process thereof is complicated because Thin Film Transistors (TFTs) and electrodes must be formed using a semiconductor manufacturing process.
In contrast, PDPs are advantageous in that the manufacturing process thereof is simple, and is thus suitable for the implementation of large-sized screens, but are disadvantageous in that the power consumption thereof is high, the discharge and light emission efficiency thereof are low, and the price thereof is high.
New types of display devices capable of overcoming the disadvantages of the above-described FPDs have been developed. Recently, there has been proposed a display device that can display images through micro Spatial Light Modulators (SLMs), which are formed for respective pixels using Micro Electromechanical Systems (hereinafter referred to as “MEMSs”), which are based on an ultra-micro machining technology.
SLMs are converters that are configured to modulate incident light into a spatial pattern corresponding to an electrical or optical input. The incident light may be modulated in phase, intensity, polarization or direction. Optical modulation can be achieved using several materials that have several electro-optic or magneto-optic effects or material that modulates light through surface deformation.
FIG. 1 is a perspective view of a prior art open hole-based diffractive optical modulator.
Referring to the drawing, the prior art open hole-based diffractive optical modulator includes a substrate 101.
The open hole-based diffractive optical modulator further includes an insulating layer 102 that is formed on the substrate 101.
The open hole-based diffractive optical modulator further includes a lower reflective part 103 that is formed on part of the insulating layer 102 and is configured to reflect incident light that passes through the holes 106aa to 106nb of upper reflective parts 106a to 106n and the spaces between the upper reflective parts 106a to 106n. 
The open hole-based diffractive optical modulator further includes a pair of side support members 104 and 104′ that allow the lower reflective part 103 to be interposed therebetween, and are formed on the surface of the substrate 101 to be spaced apart from each other.
The open hole-based diffractive optical modulator further includes a plurality of laminate support plates 105a to 105n that have side portions supported by the pair of side support members 104 and 104′, are spaced apart from the substrate 101, have central portions movable upward and downward, have holes (not shown) corresponding to the holes 106aa to 106nb formed in the upper reflective parts 106a to 106n at the central portions thereof, and constitute an array.
The open hole-based diffractive optical modulator further includes the upper reflective parts 106a to 106n that are respectively formed at the central portions of the laminate support plates 105a to 105n, have the holes 106aa to 106nb at the centers thereof, so that they reflect some incident light and allow the remaining incident light to pass through the holes 106aa to 106nb, and constitute an array.
The open hole-based diffractive optical modulator further includes a plurality of pairs of piezoelectric layers 110a to 110n and 110a′ to 110n′ that are formed over the laminate support plates 106a to 106n, are spaced apart from each other, are placed over the side support members 104 and 104′, and are configured to move the laminate support plates 106a to 106n upward and downward.
In the piezoelectric layers 110a to 110n and 110a′ to 110n′, when voltage is applied to the lower electrode layers 110aa to 110na and 110aa′ to 110na′, the piezoelectric material layers 110ab to 110nb and 110ab to 110nb′ and the upper electrode layers 110ac to 110nc and 110ac′ to 110nc, the central portions of the laminate support plates 105a to 105n move upward and downward due to the contraction and expansion of the piezoelectric material layers 110ab to 110nb and 110ab′ to 110nb′. Accordingly, the upper reflective parts 106a to 106n move upward and downward.
Meanwhile, when light is incident on the upper reflective parts 106a to 106n of the open hole-based diffractive optical modulator, the upper reflective parts 106a to 106n reflect part of the incident light and allow the remaining part of the incident light to pass through the holes 106aa to 106nb, and the lower reflective part 103 reflects light that has passed through the holes 106aa to 106nb of the upper reflective parts 106a to 106n. 
As a result, the light reflected from the upper reflective parts 106a to 106n and the light reflected from the lower reflective part 103 forms diffracted light having several diffraction orders. The intensity of the diffracted light is highest when the difference in height between the upper reflective parts 106a to 106n and the lower reflective part 103 is an odd multiple of λ/4 where λ is the wavelength of the incident light, and is lowest when the difference in height between the upper reflective parts 106a to 106n and the lower reflective part 103 is an even multiple of λ/4.
FIG. 2 is a partial sectional view of the open hole-based diffractive optical modulator, which is taken along line A-A′ of FIG. 1 and shows the sections of first and second upper reflective parts 106a and 106b. 
In FIG. 2, when the interval between the upper reflective parts 106a and 106b and the lower reflective part 103 is configured to be a first interval
      λ    4    +            n      ⁢                          ⁢      λ        2  (where λ is the wavelength of incident light and n is an integer), the intensity of light is lowest.
Furthermore, when the interval between the upper reflective parts 106a and 106b and the lower reflective part 103 is a second interval
      n    ⁢                  ⁢    λ    2(where λ is the wavelength of incident light and n is an integer), the intensity of light is highest.
Meanwhile, in order to obtain the highest intensity of light, the first upper reflective part 106a, indicated by solid lines, must be displaced by I1 or L1, while the second upper reflective part 106b, indicated by solid lines, must be displaced by I2 or L2.
However, there are many cases where the upper reflective parts 106a and 106b are located not at initial positions, indicated by solid lines, but at positions indicated by dotted lines, after the passage of time, due to frequent upward and downward movement even when voltage is not applied to the piezoelectric layers 110a, 110a′, 110b and 110b′. In this case, in order to obtain the lowest intensity of light or the highest intensity of light, the first reflective part 106a must be displaced by I1′ or L1′, while the second reflective part 106b must be displaced by I2′ or L2′.
As a result, there is variation in the displacement of the upper reflective parts 106a to 106n, which are required to represent the lowest intensity of light or the highest intensity of light. Only when the variation in the displacement is compensated for can the desired accurate lowest intensity of light or the desired accurate highest intensity of light be obtained.